Re: [R] [PS] rmultinomial() function

From: Ben Fairbank <BEN_at_ssanet.com>
Date: Wed, 19 Mar 2008 09:35:47 -0500

Mary --

The dmultinomial function (try ?Multinomial, noting that it is an upper case M) has a "log" option, which, if set to TRUE, returns logarithms of probabilities, but that is for computing probabilities, not generating samples. Perhaps the "long" you referred to is a misprint for "log?"

In any case, try ?Multinomial, and give rmultinom() another try. Note, however, that its output gives the _number_ of each of the sampled items produced in a single sample, not the sequence of draws. If you need the sequence, then I think that the reply from Erik Iverson tells you how best to proceed.

Ben

-----Original Message-----
From: r-help-bounces_at_r-project.org [mailto:r-help-bounces_at_r-project.org] On Behalf Of Mary Black
Sent: Tuesday, March 18, 2008 5:29 PM
To: r-help_at_r-project.org
Subject: [PS] [R] rmultinomial() function

After scouring the online R resources and help pages, I still need clarification on the function rmultinomial(). I would like to create a vector, say of 100 elements, where every element in the vector can take on the value of 0, 1 or 2, and where each of those values have a specific probability. ie. the probability a given element in the vector = 0 is 0.06, 1 = 0.38, 2 = 0.56 (probabilities sum to 1). Can I use rmultinomial() function to do this?

The following code does not seem to produce the result I need, but this sort of code is all I could find the R "help" pages:  

> rmultinomial(100,c(0.06,0.38,0.56))

[1] 3 29 68
> rmultinomial(100,c(0.06,0.38,0.56),long=TRUE)
  [1] 3 3 2 2 3 2 3 3 2 3 2 3 3 3 2 2 3 3 2 3 1 2 3 3 3 3 3 3 2 3 3 2 3 3 3 2 3 2 2 3 2 3 2 3 3 3 2 1 3 3 1
 [52] 2 3 2 2 3 3 2 2 2 1 3 3 2 3 3 3 3 2 3 3 3 3 2 3 2 3 3 2 3 3 2 3 3 2 3 2 3 3 2 3 3 3 2 3 2 2 2 2 2

Also, I don't really understand the difference between the default long=FALSE and long=TRUE. The R "help" simply states that you use "long TRUE to choose one generator, FALSE to choose another one"; however I could not find any documentation that described what the difference between those generators is. Any clarification would be greatly appreciated!

Thanks,

Mary



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https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code. Received on Wed 19 Mar 2008 - 14:56:25 GMT

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